The shutdown of large power stations and setting up at the same time of an increasing number of renewable energy plants and the associated growing proportion of fluctuating power supply causes increasing deterioration of mains stability and increases the risk of black-outs. The inventors have recognized that this presents substantial challenges not only to distributing mains operators, but also transmission mains operators. Instabilities of that kind, as caused by fluctuating power supply, in a distributing mains can penetrate the transmission mains by way of the regulating transformer coupled to these mains and are therefore to be avoided as far as possible.
EP 1 134 867 B1 describes a system protection centre for an electrical energy supply mains and a method for assessing the stability of an electrical energy supply mains, where the mains includes a plurality of substations, buses and lines and a system protection centre. This method includes the following steps: transmitting substation data with respect to the state of switches of at least one substation to the system protection centre; measuring phasor data for voltages and currents at a plurality of locations of the mains; transmitting the phasor data to the system protection centre; the system protection centre determines from the phasor data and the substation data at least one power, voltage or impedance stability margin value as a measure of the stability of the supply mains; the system protection centre determines one or more control commands; the system protection centre transmits the control commands to the at least one substation; the substation executes the control commands; the system protection centre determines mains state data; the system protection centre transmits the mains state data to an energy management system; and the energy management system controls power generation and power flow within the mains in accordance with the mains state data.
In this method, the energy management system collects the substation data from a plurality of substations. The energy management system controls power generation and power flow within the mains on the basis of this collected data in that it transmits control commands to the substations. In this method, the system protection centre receives phasor data from phasor measuring units which are located at supply lines of several substations and/or at branch points along transmission lines. In this method, phasor data of phasor measuring units, which are distributed over a large geometric area of hundreds of kilometres, are collected. In this method, determination of the stability of the supply mains is carried in accordance with a method from VU, K. ET AL: “USE OF LOCAL MEASUREMENTS TO ESTIMATE VOLTAGE-STABILITY MARGIN” (POWER INDUSTRY COMPUTER APPLICATIONS (PICA), IEEE, 12-16 May 1997).
VU, K. ET AL: “USE OF LOCAL MEASUREMENTS TO ESTIMATE VOLTAGE-STABILITY MARGIN” (POWER INDUSTRY COMPUTER APPLICATIONS (PICA), IEEE, 12-16 May 1997) describes a method that determines the stability of a transmission mains. This method uses a load model for a load rail (“load bus”) of the transmission mains and treats the rest of the transmission mains as a Thévenin equivalent. In this method the load model defines the load impedance by: Zapp=u(t)/i(t) and uses |Zapp|>|ZThev| as stability criterion, wherein ZThev is the Thévenin impedance.